Position control arrangement, especially for a surveying instrument, and a surveying instrument

ABSTRACT

In a position control arrangement for controlling the rotational position of a movable unit, especially for a surveying instrument, an electric motor is arranged to rotate the movable unit around an axis of rotation, and there are controllers for enabling the motor to stop the movable unit in a desired rotational position. The motor is a direct drive motor, the shaft of which forms the axis of rotation for the movable unit, and the motor is arranged to selectively operate in either a first, normal mode for rotating the movable unit to a desired position, or in a second, friction mode for providing resistance to a forced rotation of the movable unit from a predetermined position to a new position. A control unit detects the presence of a forced rotation and automatically changes the mode of operation in response thereto.

This application is a divisional of U.S. application Ser. No.11/084,797, filed Mar. 18, 2005, which is a continuation-in-part (CIP)of International Application No. PCT/SE2003/001375, filed Sep. 4, 2003,claiming priority of Swedish Application No. SE 0202793-6, filed Sep.20, 2002, the entire contents of each of these applications areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a position control arrangement, and inparticular to a position control arrangement for controlling theoperation of a motor adapted to rotate a movable unit, which may be anoptical movable unit of the type forming part of a surveying instrument,including a survey instrument known as a total station.

BACKGROUND OF THE INVENTION

In surveying instruments, especially in so-called total stations, it isoften required to position a movable unit in the instrument by rotatingthe movable unit around a horizontal or a vertical axis. The rotationaround an axis is normally made via a gear arrangement, either by handfor a manual instrument or by means of an electric motor that isconnected to the gear arrangement for an automated instrument. Such agear arrangement needs to be of very high precision in order to serveits purpose, especially when the instrument is intended to automaticallymove from one position to another, for instance by means of a trackingarrangement. Such an instrument with a suitable gear arrangement isexpensive. Also, in order to meet strict precision requirements, apreloading of the gear arrangement is required. Due to the resultingbacklash and hysteresis in the gear arrangement there is a need, in anautomated instrument, to have angle sensors both on the motor and on themovable unit in order to obtain a sufficiently stable and exact servoloop for controlling the motor.

The presence of a gear arrangement between the motor and the movableunit prevents a forced movement by hand of the movable unit to a newposition, since the gears will block such a movement. In order, however,to allow a forced movement by hand of the movable unit to a new positionit has been common to provide a friction clutch between a geararrangement and the movable unit, thus allowing the movable unit to movewhile the gear arrangement remains in position.

By using a gear arrangement and two angle sensors as well as a frictionclutch at each axis around which the movable unit is to be moved by amotor, the surveying instrument becomes relatively complicated andexpensive. There is therefore a need for an improved design in theserespects.

As regards general rotation of a movable unit in a surveying instrumentit is previously known through US 2002/0005944 A1 to provide an opticaltransmitter, where a head assembly is rotated around a vertical axis bymeans of a direct drive electric motor, and where there are means toemit light signals to a distant receiver at certain rotational positionsfor the head assembly. For an arrangement of this kind there is,however, no need or intention to stop the head assembly in a certainrotational position. Also, there is no need to make possible a forcedrotation of the head assembly.

OBJECTS OF THE INVENTION

It is an object of the invention to provide an improved position controlarrangement that is simpler and that is cheaper to manufacture. It isanother object to provide a position control arrangement that is highlyaccurate in use, while being simple to operate. A further object is toprovide an arrangement that requires less space. A still further objectis to provide an improved surveying instrument.

SUMMARY OF THE INVENTION

The embodiments of the invention include a method and apparatus forcontrolling angular movement of a movable unit between an existingangular position and a desired angular position. The method comprisesderiving an error signal representing an angular difference between theexisting position and the desired position of the movable unit,monitoring a first voltage caused by incident forced angular movement ofthe movable unit, and generating a second voltage when either said errorsignal or said first voltage exceeds a respective predeterminedthreshold amount. The second voltage represents an angular forcesufficient to retard the angular movement of the movable unit caused byincident force and is generated by calculating from the rate of changeof the angular movement caused by incident force a value defining theangular force to oppose such forced angular movement. The angular forceis applied to the movable unit in opposition to the angular movement ofthe movable unit caused by the incident force but, in one embodiment,only if it is less than a predetermined value. Such an opposition forcemay not be generated when it is determined that the velocity of theangular movement of the movable unit caused by incident force is lessthan a predetermined threshold amount. The second voltage drives adirect-drive electric motor adapted to rotate the movable unit.

The apparatus of an embodiment of the invention includes a motor, whichmay be a direct-drive electric motor, for controllably applying torqueto the movable unit about an axis of rotation, a position detector todetect actual rotational position of the movable unit, and a controlcircuit for selectively operating the motor in one of a normal mode anda friction mode. In the normal mode the control circuit is responsive tothe difference between the actual angular position of the movable unitand a desired angular position of the movable unit. In controlling themotor to rotate the movable unit to the desired rotational position thecontrol circuit attempts to drive the difference to zero. In thefriction mode the control circuit controls the motor to apply a torqueresisting a change of rotational position of the movable unit caused byincident force while permitting the incident force to rotate the movableunit. The control circuit is operative selectively to change operationof the motor from the normal mode to the friction mode when incidentforce causes rotation of the movable unit. The control circuit monitorsthe angular velocity of the movable unit during a forced rotation andreturns the operating mode of the motor to the normal mode when theforced angular velocity of the movable unit is lower than apredetermined threshold.

The control system of an embodiment of the invention may constitute acomputer system consisting of a processor and a computer recordingmedium including computer executable code executable by said processor.The computer executable code includes code for deriving error datarepresenting an angular difference between the existing position and thedesired position of the movable unit, code for monitoring datarepresenting a voltage value derived from the application of incidentforce to rotate the movable unit, code for comparing the error data andvoltage value data to respective predetermined threshold amounts, andcode for generating a control signal when either the angular differenceerror or the voltage value exceeds the respective predeterminedthreshold amount. The control signal is applied to cause a change in theoperating mode of the motor from a normal mode to a friction mode. Withrespect to friction mode, the computer system includes code fordetermining and monitoring a rate of change in the angular movement ofthe movable unit caused by incident force and determining therefrom avelocity of such forced angular movement of the movable unit. Additionalcode may be implemented for calculating from the rate of change ofangular movement a value representing a resisting angular force tooppose the forced angular movement of the movable unit. A direct-driveelectric motor may be used to apply the resisting angular force to themovable unit in opposition to angular movement of the movable unitcaused by incident force. The computer system determines when theangular velocity of the movable unit caused by incident force is lowerthan a predetermined threshold and consequently returns the operatingmode of the motor to its normal mode.

By providing a direct-drive electric motor the need for a geararrangement is eliminated, and by making it possible to selectivelyoperate the motor in a normal mode or in a friction mode, the need for aconventional friction clutch for allowing a forced rotation of themovable unit is eliminated.

By providing means for automatically switching the mode of operation forthe motor in response to the presence of an external force influencingthe movable unit, a simple operation is made possible, since theoperator does not have to be concerned about selecting a mode ofoperation.

In a further embodiment a friction control unit is adapted to control,in friction mode, the magnitude of resistance to forced rotation. Thefriction control unit is also arranged to limit such resistance to aselected value. In this way good precision can be obtained when manuallyshifting the position of the movable unit.

The elimination of the gear arrangement also provides a more quiet andquick operation when shifting the movable unit from one position toanother.

The control system of embodiments of the invention is particularlyuseful in connection with the surveying instrument known as a totalstation. In total stations generally the movable unit is within a fixedhousing of the survey instrument and is mounted for rotation back andforth with respect to the housing in both horizontal and verticaldirections. The movable unit contains the survey optical equipment. Twomotors may be used each having a shaft defining the axis of rotation ofthe movable unit in one of the horizontal and vertical directions. Itwill be understood that a survey instrument in which a single motor isadapted for use in driving the movable unit about mutually perpendicularaxes is within the scope of the present invention. Control over eachmotor may be exercised according to the foregoing method and apparatus,including by a programmed computer system. The motors may bedirect-drive electric motors.

Further objects, features and advantages of the invention will becomeapparent from the following description of a preferred embodiment, withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of a surveying instrument embodying theinvention,

FIG. 2 is a schematic sectional view of a motor arrangement used in aninventive position control arrangement,

FIG. 3 is a block diagram illustrating an inventive position controlarrangement, and

FIG. 4 is a flowchart illustrating an inventive position control methodimplemented in a computer.

DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 illustrates one application for the control system of anembodiment of the present invention. In FIG. 1 there is illustrated afront view of a surveying instrument 1 of the type commonly referred toas a total station. A movable unit 2, comprising optical equipment ofthe total station, as represented by a lens 3, is supported in a housing4 in such a manner that it may be pivoted relative to the housing 4around an essentially horizontal axis 5, as indicated by double arrow 6.The housing 4 comprises a base 7 by means of which it is supported on astand 8 in such a manner that it is rotatable relative to the stand 8around an essentially vertical axis 9, as indicated by double arrow 10.Thus, by rotating the movable unit 2 around the two axes 5 and 9 themovable unit 2 can be oriented in any desired position for the purposeof carrying out an intended surveying operation. For movement of themovable unit 2 around the axis 5 there is provided a motor arrangement11 in the housing 4, and for movement of the housing 4 and the movableunit 2 around the axis 9 there is provided a similar motor arrangement12 in the stand 8. Each of the two motor arrangements 11, 12 is includedin a position control arrangement related to a respective axis 5 and 9,as described below, and each of the motor arrangements 11 and 12 mayinclude a direct-drive electric motor. The surveying instrument 1 isintended to be supported via the stand 8 on a tripod, or similarequipment, in a conventional manner.

The two motor arrangements 11 and 12 are similar in design, and for thesake of simplicity only the motor arrangement 12 will be describedbelow, while referring to FIG. 2, where portions of the stand 8 havebeen omitted for the sake of clarity.

As shown in FIG. 2, a drive shaft 13 of the motor in one embodiment isrotatably supported by a bearing 14 secured in the upper portion of thestand 8 and engages non-rotatably the base 7 of the housing 4, so thatthe base 7 can be rotated by means of the drive shaft 13 in order toindirectly rotate the movable unit 2 around the axis 9. In thisembodiment the drive shaft of the motor defines the rotational axis ofthe base. Also in the present embodiment, the drive shaft 13non-rotatably carries the rotor 15 of the motor, while the stand 8non-rotatably carries the stator 16 of the motor.

In order to be able to decide the actual rotational position of thedrive shaft 13, and thus indirectly of the movable unit 2, there isprovided a position detecting arrangement 17, which may include an anglesensor, that is arranged to detect a difference between the relativepositions of the stator 16 and the rotor 15. In the embodiment shown,the position detecting arrangement 17 comprises two detecting units 18and 19, which are located diametrically opposed around the drive shaft13. These two detecting units 18, 19 are of essentially similar design.In each of them a light emitter 20 secured on the stator 16intermittently directs light towards a reflector 21 secured on the driveshaft, and from said reflector light is reflected towards a sensorsecured on the stator 16. On its way from the reflector 21 to the sensor22 the light passes through a transparent disc 23 non-rotatably securedon the drive shaft 13. The disc 23 is provided with a pattern thatcauses specific position signals to be generated in the sensor 22 as thedrive shaft 13 rotates. By using a high rate of signal production a highdegree of position accuracy can be obtained by the position detectingarrangement 17.

The two detecting units 18, 19 can suitably be arranged to have theirreflected light pass through radially different regions with mutuallydifferent patterns of the transparent disc 23 in order to allow improvedaccuracy of the position detecting arrangement 17.

It will be noted from FIG. 2 that the two detecting units 18 and 19 ofthe position detecting arrangement 17 are located in an essentiallyclosed environment within the motor arrangement 12 and therefore arewell protected. The number of detecting units 18, 19 as well as theirlocation and their design can of course be varied as desired.

For controlling the motor arrangements 11 and 12 in order to obtain adesired position for the movable unit 2 there is provided a positioncontrol arrangement for each of the two motor arrangements. Theseposition control arrangements are similar in design, and for the sake ofsimplicity only the position control arrangement related to the motorarrangement 12 will be described below. It will be understood that anembodiment of the invention consisting of a single position controlarrangement having two channels, one for each motor arrangement, iswithin the scope of the present invention.

Referring now to FIG. 3, there is disclosed schematically in blockdiagram form an inventive position control arrangement 30 related to themotor arrangement 12. There is provided an input means 31, by means ofwhich the position control arrangement 30 receives information about theintended position or attitude for the movable unit 2 that is operated bythe motor arrangement 12. Such intended position information can beprovided to the input means via input by an operator, for example via ajoystick or other means, or automatically by one or more units in asurveying system. As an example, the surveying system might comprise atracking arrangement for enabling the movable unit to automaticallyfollow a target. The target may transmit information concerning itslocation directly to suitable input means at 31. Several othertechniques for inputting data concerning an intended or desired positionfor the movable unit 2 are previously known and need not be described infurther detail here.

Signals from the input means 31 are, in normal operation, fed to afilter unit 32. The filter 32 acts to smooth the output from the inputmeans 31. That is, for a large change in intended position relative tothe actual position of the movable unit, the output from the filter 32ramps up or down at a substantially constant acceleration anddeceleration. It will be understood that the smoothing function can beimplemented within the input means 31, as desired, without departingfrom the scope of the present invention. The output from the filter 32is fed to an adding unit or comparator 33, the output from which is fedto a regulator 34, which may be a Kalman filter, that controls thevoltage to be applied to drive the motor 12. In response to the voltageinput the motor 12 brings about a movement of the movable unit 2 (inthis case via the housing 4 and around the axis 9), which in turn causesthe position detecting arrangement 17 to generate a position signalcorresponding to the actual position of the movable unit 2. In aconnection between the position detecting arrangement 17 and the addingunit 33 there is provided a sign change unit 35 that changes the sign ofthe position signal fed to the adding unit 33. In this way there isprovided a control loop for governing the motor 12 to an intendedrotational position. As long as there is a difference between thesignals received in the adding unit 33 from the filter unit 32 and thesign change unit 35, the regulator 34 will receive a signal and output avoltage, causing the motor 12 to run. The purpose of the loop is todrive the difference to zero at which point the regulator 34 willreceive no signal and the motor will automatically stop. The motor canbe controlled so as to turn the movable unit in either direction. Thedirection of rotation will depend upon the sign of the control erroroutput from the unit 33.

There is also provided a control unit 36, which receives input signalsfrom the input means 31, the position detecting arrangement 17, theregulator 34, and the adding unit 33. On the basis of these signals thecontrol unit 36, which may be part of a computer system, decides if theposition control arrangement 30 should operate in a normal mode, asdisclosed above, or in a friction mode, to be described below. To thisend the control unit 36 controls a switch arrangement 37 comprising twoswitches 38 and 39, which may be digital switches, included as part ofthe position control arrangement 30. The switch 38 operates between theinput means 31 and the filter unit 32 and is capable of connecting thefilter unit 32 either with the input means 31 (normal mode, shown with afull line) or with the position detecting arrangement 17 (friction mode,shown with a dashed line). The switch 39 operates between the regulator34 and the motor 12 and is capable of connecting the motor 12 eitherwith the regulator 34 (normal mode, shown with a full line) or with thesign change unit 35 via a friction control unit 40 (friction mode, shownwith a dashed line).

If, during normal operation, an operator wants to quickly set themovable unit 2 into a new attitude or position, the operator canmanually force the movable unit 2 to rotate around either axis toward adesired attitude. In accordance with the friction mode of operation,such action will cause the motor to apply a force resisting the manualmovement of the movable unit. According to an embodiment of theinvention, when the movable unit 2 is manually rotated in such a way,the position detecting arrangement 17 will detect a large and quickchange in position of the movable unit 2. That is, the output from theposition detecting arrangement 17 can represent an unusually largeacceleration in the rotational movement of the movable unit. This cancause an abnormality in the output error signal from the adding unit 33which, in turn, will cause the output voltage from the regulator 34 tobe unusually large. The unusually large voltage value is applied to themonitoring control unit 36. In an embodiment of the invention, thecontrol unit 36 compares the voltage signal input from the regulator 34to a predetermined threshold value. If the received voltage valueexceeds the threshold value, a control signal is output by the controlunit 36 which activates the switches 38 and 39 to connect the motor 12to the friction control unit 40, thereby switching the system tofriction mode. As an example, if the value of the output voltage fromthe regulator 34 is determined to be about twice what that outputvoltage is under normal operation then the control unit 36 determinesthat the system should no longer be in normal mode and should beswitched to friction mode.

It should be understood that forced manual turning of the movable unitmay also cause a large difference to appear between the signal valuesrepresenting the actual position of the movable unit 2, as determined bythe position detectors at 17, and the signal values output from theinput unit 31, which represent the previous intended position for themovable unit 2. An unusually large difference between those signalvalues will provoke an unusually large error signal or error angle valueto be output from the adding unit 33. An output error signal from theadding unit 33 representing this large difference between the positionvalues input thereto is fed to the control unit 36. The control unit 36compares the received error signal to a predetermined threshold and maydetermine as a result that the operation mode is no longer normal. Ifso, a control signal is output from the control unit 36 to cause theswitches 38 and 39 to shift from the normal mode position, representedby full lines, to the friction mode position, represented by dashedlines. As a result, both the filter unit 32 and adding unit 33 will nowreceive input signals directly from the position detecting arrangement17 concerning the actual position of the movable unit 2. Also, the motor12 will become connected to the friction control unit 40 rather than tothe regulator 34, the output from which will be substantially zero undersuch conditions.

It is important to note that either a large error signal from the addingunit 33 or a large voltage output from the regulator 34 may trigger thecontrol unit 36 to switch the system from the normal mode to thefriction mode. The values required to cause a change in operating modeare significantly larger than the largest error signal produced by theadding unit 33 and the largest voltage output from the regulator 34obtained when the system is operating in the normal mode. For example,the threshold output voltage of the regulator 34 for causing the controlunit 36 to switch the system from the normal mode to the friction modemay be about twice the largest voltage output occurring from theregulator 34 during operation in the normal mode. Alternatively, themagnitude of the error signal produced by the adding unit 33 sufficientto cause the control unit 36 to switch the system from normal mode tothe friction mode may be in a range of from 10 to 50 times the largesterror signal output from adding unit 33 expected during the normal modeof operation.

An advantage of setting a threshold for causing the control unit 36 toswitch from the normal mode to the friction mode using either the outputof the adding unit 33 or the output of the voltage regulator 34, asdescribed above, is that an accidental switching to the friction mode isprevented even when a large change of intended position is input to theinput means 31. It should be understood by a person of ordinary skill inthe art that smoothing the output of the input means 31 is also animportant feature in preventing this accidental switching.

In response to the forced movement experienced by the movable unit 2 thefriction control unit 40 causes the motor 12 to produce a torqueresisting the forced movement. The magnitude of this torque may bedecided as a function of the temporary acceleration and speed of theforced movement, so as to adapt the magnitude of the torque to the kindof movement performed. It will be understood that the character of theresisting torque can be selected in many ways and might easily bechanged by a change in software in the friction control unit 40. In anembodiment of the invention the magnitude of the torque allowed by thefriction control unit 40 is limited to less than a predetermined valuein order to protect the motor 12.

The control unit 36 determines the rate of change of angular movement ofthe movable unit caused by incident force and from that calculates thevelocity of resulting angular movement of the movable unit. So long asthese values exceed predetermined threshold values, the control unit 36maintains the system in friction mode. Once the forced movement of themovable unit 2 ceases, for example after reaching a manually fixedposition, the control unit 36 will, on the basis of position informationreceived from the position detecting arrangement 17, notice that therate of change of position or angular velocity of the movable unit havebecome quite small, i.e. have dropped below the predetermined thresholdvalues. The control unit determines therefore that the position controlarrangement 30 now in fact should operate the system in normal mode. Asa result, the switches 38 and 39 are operated to reset the arrangement30 from friction mode to normal operating mode (shown in full lines inFIG. 3) as described above.

When the control unit 36 causes the switch from the friction mode to thenormal mode, the control unit 36 resets the input means 31 with themanually fixed position achieved while the system was operating in thefriction mode. This resetting is performed to prevent the movable unit 2from being returned to the previously set intended position once thesystem has returned from the friction mode to the normal mode ofoperation.

The friction control unit 40 can comprise a derivating module 41 adaptedto calculate from the signals provided by the position detectors 17 therate of change in the angular position of the movable unit 2 and itsrotational direction induced, for example, by a manually forced rotationof the movable unit. The forced rate of change in angular position isfed to a proportioning module 42 which then calculates those countertorque direction and values which are related to the direction of andthe rates of change of angular position caused by the manual action. Alimiting module 43 for limiting the value of the resistive torque to beapplied is provided to receive the output of the proportioning module42. The limiting module may include a digital filter which passes onlythose values corresponding to an output voltage representing a countertorque that is less than a predetermined value. The counter torquegenerated in friction mode resembles a viscous friction from aconventional friction device such as a slipping clutch or similararrangement, that is, the faster the movable unit 2 is turned the morebraking is perceived. The modules 41-43 of the friction control unit 40can be implemented in software.

It will be understood that if a survey instrument, for example, has twodrive motors 11 and 12, one for each of the horizontal and verticalaxes, each of the motors may, if desired, have a dedicated positioncontrol arrangement 30 as described above. Alternatively, a simpler andmore compact embodiment comprising fewer parts may be provided in whicha single position control arrangement having dual channels is used tocontrol the operation of each of the two motors.

In order to obtain a good precision it is desirable to use an angleposition sampling rate of about 1000 Hz and to let the light emitters 20generate very short flashes of light. Each flash could have a durationof only a few microseconds. The angular velocity of each motorarrangement 11, 12 might have a maximum value of about 1 or 2 rad/s, andthe maximum angular acceleration might be around 3 rad/s². It is alsodesirable that the power consumption of each motor is as low aspossible, since the motors normally will be battery operated in order toprovide flexible use of the surveying instrument.

For precision control of a motor it is desirable to take into accountthe inertia of the parts to be moved, in order to monitor the torque sothat adequate acceleration and retardation is obtained to provide quickand accurate positioning.

It will be clear by the above description and by the flowchart of FIG. 4that a method for controlling angular movement of a movable unit can beimplemented in a computer system including a processor and a computerrecording medium including computer executable code executable by theprocessor.

As shown in FIG. 4, an implemented method would include the steps ofmonitoring an existing angular position of a movable unit in step S10,introducing a desired angular position for the movable unit in step S20,comparing the existing angular position and the desired angular positionto derive an angular difference in step S30, deriving an error signalbased on the angular difference in step S40, and monitoring in step S50a first voltage resulting from angular movement of the movable unitcaused by incident force on the movable unit.

When the error signal or the first voltage signal are not aboverespective thresholds in step S60, the implemented method continues themovement of the movable unit until the error signal is zero in step S70.This is when the desired position is reached and the movable unit isstopped at this position in step S80.

When the error signal or the first voltage signal are above therespective thresholds in step S60, the implemented method goes on tomonitoring the rate of change in angular movement of the movable unitand determining therefrom a velocity of angular movement of the movableunit in step S90, calculating from the rate of change of angularmovement a torque value sufficient to oppose the angular movement instep S100, and limiting the torque to a threshold value and applying thetorque value as a second voltage to the movable unit in opposition tothe angular movement of the movable unit in step S110.

Further, in step S120, the implemented method goes on to cease togenerate the second voltage when the velocity of angular movement of themovable unit is less than a threshold value and resets the desiredangular position with the actual angular position. In step S130 themovable unit is stopped at this actual angular position.

It will also be understood that within the scope of the inventionvarious other changes than the ones mentioned can be made to thedisclosed embodiment, and that the scope of the present invention isdefined by the following claims:

1. An apparatus for controlling angular movement of a movable unitbetween an existing angular position and a desired angular position,comprising an electric direct drive motor adapted to rotate said movableunit, said motor having a shaft defining an axis of rotation of themovable unit and being selectively operable in one of a normal mode forrotating said movable unit to a desired position and a friction modepermitting incident force to rotate said movable unit from said desiredposition to a new position; a mode control unit to enable said frictionmode of said motor upon the application of said incident force; and afriction control unit connected to said motor in said friction mode, anddisconnected from said motor in said normal mode.
 2. The apparatus ofclaim 1 in which said friction control unit is operative uponapplication of said incident force to cause said motor to resistrotation of said movable unit at a predetermined magnitude ofresistance.
 3. The apparatus of claim 1 comprising, in addition, aposition detector to detect the angular position of the movable unit. 4.A controller for controlling a direct drive motor having a rotatingshaft, the controller comprising: input means for receiving an intendedrotational position of said rotating shaft of said motor; positiondetecting means for detecting an actual rotational position of saidrotating shaft; control means for controlling said rotating shaft torotate from said actual rotational position of said rotating shaft tosaid intended rotational position of said rotating shaft; and frictioncontrol means for causing said motor to produce a torque resisting amanually forced rotation of said rotating shaft while said rotatingshaft is manually rotated to a new rotational position, wherein saidcontrol means automatically connects said motor to said friction controlmeans in response to a detection of said manually forced rotation ofsaid rotating shaft to said new position, and automatically disconnectssaid motor from said friction control means when said new rotationalposition is reached.
 5. An apparatus for controlling rotational positionof a movable unit about an axis of rotation, comprising: a motor forcontrollably applying torque to move the movable unit about the axis; aposition detector to detect actual rotational position of the movableunit; a control circuit for selectively operating the motor in one of anormal mode and a friction mode, wherein (1) in the normal mode thecontrol circuit is responsive to a difference between actual rotationposition and an intended rotational position of the movable unit forcontrolling the motor to rotate the movable unit to the intendedrotational position, and (2) in the friction mode the control circuitcontrols the motor to apply a torque resisting change of rotationalposition of the movable unit while permitting incident force to rotatethe movable unit.
 6. The apparatus of claim 5, wherein the controlcircuit is operative to selectively change operation of the motor fromthe normal mode to the friction mode when incident force causes rotationof the movable unit.
 7. The apparatus of claim 5, wherein the controlcircuit comprises a mode control unit responsive to rate of change ofactual rotational position caused by incident force for selectivelychanging operation of the motor from the normal mode to the frictionmode.
 8. The apparatus of claim 7, wherein the control circuit comprisesa friction control unit responsive to the position detector fordetermining a magnitude of the torque applied in the friction mode. 9.The apparatus of claim 8, wherein the friction control unit determinesthe magnitude of the torque applied in said friction mode as a functionof acceleration of incident forced rotation of the movable unit.
 10. Theapparatus of claim 9, wherein the friction control unit determines themagnitude of the torque applied in the friction mode as a function ofvelocity of incident forced rotation of the movable unit.
 11. Theapparatus of claim 8, wherein the friction control unit comprises aderivating module responsive to the position detector for determining arate of change of angular position of the movable unit in the frictionmode.
 12. The apparatus of claim 11, wherein the friction control unitcomprises a proportioning module for determining torque required in thefriction mode to resist change of rotational position of the movableunit while permitting incident force to rotate the movable unit.
 13. Theapparatus of claim 12, wherein the friction control unit comprises alimiting module for limiting the torque applied by the motor to themovable unit in the friction mode.
 14. The apparatus of claim 8 furthercomprising a switch, the mode control unit controlling the switch toconnect the motor to the friction control unit in the friction mode andto disconnect the motor from the friction control unit to enable thenormal mode.
 15. A survey instrument comprising: a housing; a movableunit rotatable back and forth around a first axis relative to saidhousing, and including survey optical equipment; and a first positioncontrol system for controlling the angular position of said movableunit, said first position control system comprising a first motor torotate said movable unit about said one axis and a control circuitselectively to operate said first motor in one of a normal mode, inwhich said first motor rotates said movable unit to a desired position,and a friction mode in which said first motor applies a torque to saidmovable unit resisting change of angular position of said movable unitcaused by incident force while permitting incident force to rotate saidmovable unit.
 16. The survey instrument of claim 15 in which said firstmotor is a direct-drive electric motor having a shaft coincident withsaid one axis.
 17. The survey instrument of claim 15 comprising, inaddition, a second direct-drive electric motor having a shaft coincidentwith a second axis relative to said housing and in which said movableunit is selectively rotatable back and forth around said second axis.18. The survey instrument of claim 17 in which said one axis is ahorizontal axis and said second axis is a vertical axis.
 19. The surveyinstrument of claim 18 in which said first position control systemcontrols the angular position of said movable unit about each of saidhorizontal and vertical axes.
 20. The survey instrument of claim 19 inwhich said first motor rotates said movable unit about said horizontalaxis and said second motor rotates said movable unit about said verticalaxis.
 21. The survey instrument of claim 20 comprising, in addition,position detectors associated with each of said horizontal and verticalaxes to detect actual angular position of said movable unit relative toeach of said axes.
 22. The survey instrument of claim 21 furthercomprising a second position control system, said second positioncontrol system comprising said second motor and a control circuitselectively to operate said second motor in one of said normal and saidfriction modes.